Parkinson's disease is characterized by the selective degeneration of dopaminergic neurons in the substantia nigra of the midbrain. Therapies using replacement of dopaminergic neurons in patients with Parkinson's disease through transplantation of fetal midbrain tissue are being variously studied (see Reference 1). Grafts of fetal midbrain tissue can survive for a long period in the human brain, restore dopaminergic innervation to the striatum in patients and reduce motor symptoms with Parkinson's disease (see Reference 2).
Although transplantation is a promising treatment for Parkinson's disease, its clinical application has been limited to a few cases, because it is very difficult to obtain large numbers of human abortion fetal tissues. To overcome this problem, various candidate cells have been investigated as possible donor cells for transplantation therapy for Parkinson's disease (see Reference 3).
Meanwhile, human neural progenitor cells (hNPCs) derived from fetal midbrain tissue appear to be a good candidate cell source for transplantation because of their capacity to self-renew for long-term proliferation activity and to differentiate into dopaminergic neurons (see References 4 and 5). Therefore, for treating Parkinson's disease through dopaminergic neuron replacement (i.e., transplantation), it is very important to establish an efficient method for the proliferation or expansion of hNPCs and an effective method for the differentiation of hNPCs into dopaminergic neurons.
Methods for the differentiation of hNPCs into dopaminergic neurons known in the prior arts include a differentiation method in a medium containing ascorbic acid and dibutyryl cyclic adenosine monophosphate (db-cAMP) for 3 days (see Reference 6); a differentiation method in a medium containing BDNF (brain-derived neurotrophic factor), dopamine, and forskolin for 3 weeks (see Reference 7); and a differentiation method in a medium containing SHH (sonic hedgehog), FGF-8 (fibroblast growth facter-8), BDNF (brain-derived neurotrophic factor), and ascorbic acid for 3 weeks (see Reference 8).
However, the differentiation methods according to the prior arts do not exhibit satisfactory differentiation potential; and require long duration for differentiation. And also, economic problems are incurred from the use of a medium containing expensive excipients, such as SHH, FGF-8, etc. The technology for the proliferation of a large number of cells required for treating the patients suffering from Parkinson's disease is still insufficient. Therefore, there are still a lot of limitations in the clinical applications thereof.